1. Field of the Invention
This invention relates to the field of semiconductor integrated circuit fabrication and, in particular, to the fabrication of electrical interconnects on semiconductor integrated circuits.
2. Background Art
It is well known to use metal films in semiconductor integrated circuit technology in order to electrically couple various components formed on semiconductor wafers. Copper is the preferred metal material for forming integrated circuit metallization including small geometry high density integrated circuit metallization. It is preferred primarily because of its very low resistivity and, therefore, its small RC time constant. Low resistivity is particularly important in small geometry integrated circuit metallization because the thickness of metallization must be decreased in small geometries. However, decreases in the thickness of metallization increase resistance. Additionally, copper has a high resistance to electromigration and high thermal conductivity. However, a number of issues must be addressed before copper can be reliably used in very small integrated circuit geometries.
A serious problem with the use of copper in small circuit geometries is that copper is highly diffusive through silicon and silicon based interlevel dielectric materials. This permits copper atoms from copper interconnects to reach nearby transistors in small circuit geometries, and change the electrical properties of the transistors.
A barrier cladding layer on the surface of the copper metallization can be used to prevent the unwanted copper diffusion into the silicon. One material which effectively serves as a cladding layer on copper to prevent diffusion is titanium nitride (TiN). For example, J. Li, J. W. Mayer, in "Advanced Metallization for ULSI Applications," 1992, proceedings of the conference held Oct. 20-22, 1992, in Tempe, Ariz., sponsored by Continuing Education in Engineering, University Extension, University of California, Berkeley, Calif., U.S.A.; editors, Timothy S. Cale, Fabio S. Pintchovski; Pittsburgh, Pa., Materials Research Society, a method for forming such a barrier cladding layer formed of titanium nitride is taught.
In the method taught by Li, et al., a copper titanium alloy is heat treated in ambient ammonia or nitrogen plasma. When the alloy is heated to approximately 450.degree. C., the titanium near the surface of the film diffuses to the surface and reacts with nitrogen to form the titanium nitride layer. The titanium nitride layer formed on the metallization in this manner is an effective barrier to copper diffusion. However, the bulk resistivity of the copper titanium alloy forming the metallization is too high for the alloy to be used as metallization in integrated circuits.
Other methods for forming titanium nitride layers to act as diffusion barriers are known. For example, Gonzales in "Integrated Process for Fabricating Raised, Source/Drain, Short-Channel Transistors," issued on May 17, 1987, as U.S. Pat. No. 5,312,768, teaches a method for depositing titanium nitride on a surface by chemical vapor deposition. The titanium nitride taught by Gonzales acts as a diffusion barrier to prevent migration of dopant ions into the source, drain and gate of transistors where they would degrade the voltage characteristics of the transistor.
Additionally, Sandhu, in U.S. Pat. No. 5,252,518, issued Oct. 12, 1993, teaches the use of titanium nitride as part of metallization schemes for many large scale integrated circuit applications. For example, Sandhu teaches the use of titanium nitride as protection against spiking of aluminum contacts to silicon. However, neither the Gonzales reference nor the Sandhu reference teaches a method for using the titanium nitride material to clad copper interconnect lines and thereby prevent diffusion of copper from integrated circuit interconnect lines into adjacent silicon structures.
It is therefore an object of the present invention to provide improved metallization within high density integrated circuits.
It is a further object of the present invention to provide a method for preventing diffusion of copper from copper metallization while providing metallization of very low resistivity.
It is a further object of the present invention to provide an improved method for preventing diffusion of copper from copper integrated circuit metallization into surrounding silicon materials.
These and other objects of the present invention will become more fully apparent from the description and claims that follow or may be learned by the practice of the invention.